In a typical cellular radio system, radio or wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (in a Universal Mobile Telecommunications System (UMTS) network) or “eNodeB” (in a Long Term Evolution (LTE) network). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UEs) within range of the base stations.
In some radio access networks, several base stations may be connected (e.g., by landlines or microwave) to a radio network controller (RNC) or a base station controller (BSC). The radio network controller supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM). Universal Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using a wideband code division multiple access (WCDMA) air interface between user equipment units (UEs) and radio access network (RAN).
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. The first release for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) specification has issued, and as with most specifications, the standard is likely to evolve. The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE).
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology where the radio base station nodes are connected to a core network (via Access Gateways (AGWs)) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has what is sometimes termed a “flat” architecture including radio base station nodes without reporting to radio network controller (RNC) nodes.
A currently popular vision of the future of cellular networks includes machines or other autonomous devices communicating between each other (or with an application server) without human interaction. A typical scenario is to have sensors sending measurements infrequently, where each of the transmissions would consist of only small amounts of data. This type of communication is called machine to machine (M2M) communication in the literature, or machine-type communication (MTC), in 3GPP.
UEs in cellular systems (such as 3GPP WCDMA, LTE) are most commonly battery driven and the power consumption of these devices is therefore an important factor.
In the context of MTC, many of the devices are expected to be battery operated as well. Sensors and other devices may reside in remote locations and the number of deployed devices could be so large that it would be practically infeasible to replace or frequently recharge the batteries in these kinds of devices. Thus, it is an important goal to aim for reduction in the power consumption when considering improvements for current cellular systems.
An existing means to reduce the battery power consumption is to use discontinuous reception (DRX), a feature in which the UE's receiver is switched off except during configured periods at configured intervals.
Currently the longest specified DRX cycle lengths are 2.56 seconds and 5.12 seconds for EUTRA and UTRA, respectively. However, it would beneficial to extend the DRX cycle lengths beyond currently specified values to further reduce the battery power consumption, especially for the benefit of MTC devices where there is no possibility for interactive charging or changing of the battery on a regular basis. Although longer DRX cycle lengths naturally cause larger delays in the downlink, this is typically not a problem for delay insensitive traffic such as that generated by MTC devices. However, there are procedures such as those used by mobile devices to maintain up to date system information that may need to be aligned accordingly in case the DRX cycles are extended beyond the current limits.
In particular, it is important for a mobile device operating with DRX to maintain up-to-date system information because otherwise it cannot interact with the network in an interoperable manner. In particular, if the mobile device does not have recent system information, it must acquire the latest version of the system information prior to access, which means that it cannot access the system (e.g. transmit random access requests, etc.) before it has obtained the latest version of the system information. On the other hand, frequent acquisition of system information has an adverse impact on the battery life time. In E-UTRA networks, the information required to enable reliable communications with the network is referred to as System Information (SI) and is transmitted to the UE in a number of different System Information Blocks (SIBs) and a Master Information Block (MIB). One such element of System Information in E-UTRAN is the System Frame Number (SFN), which the UE uses to keep synchronisation with the network and which acts as a timing reference.
The SFN is defined in a Master Information Block (MIB) in a “systemFrameNumber” field which defines the 8 most significant bits of the system frame number (SFN). 3GPP TS 36.211 “E-UTRA; Physical Channels and Modulation” v11.1.0 (2012-12) [section 21, 6.6.1] indicates that the 2 least significant bits of the SFN are acquired implicitly in the physical broadcast channel (P-BCH) decoding, i.e. timing of 40 ms P-BCH transmission time interval (TTI) indicates the 2 least significant bits (within 40 ms P-BCH TTI, the first radio frame: 00, the second radio frame: 01, the third radio frame: 10, the last radio frame: 11). One value applies for all serving cells (the associated functionality is common i.e. not performed independently for each cell).
Procedures exist for notifying mobile devices operating with DRX that system information (SI) has changed. In particular, in E-UTRAN, when System Information is updated in a cell, the UEs in the cell are informed about this by a flag (called the systemInfoModification-flag) being set in the paging message sent to each UE. If the flag is set, the UEs then activate the receivers to receive and read the relevant broadcast (e.g. SIB1) accordingly. This paging is sent out to UEs during a SI modification period, which is equal to the SFN period (i.e. the period required for the SFN to cycle through the full range of SFN values) or a fraction of it, thus ensuring that all UEs have been notified of the SI change. During the following modification period, UEs read the relevant SIB and apply the new SI. According to the 3GPP Technical Specification 36.331 v11.3.0 (section 5.2.1.3), the boundaries of the modification period are defined as the SFN for which SFN mod m=0, where m is the length of the modification period in number of radio frames. Therefore, modification periods longer than the maximal SFN of 1024 are not possible.